Camshaft phase shifting mechanisms that vary the rotational angle position of a camshaft relative to a crankshaft of an internal combustion engine using pressurized hydraulic fluid to control a rotary actuator, typically using engine oil supplied by the oil pump, are known. However, such hydraulically actuated phasers can have a high oil demand, and the performance can vary depending upon the temperature of the engine oil. Further, such systems require oil flow control valves and complex passages for leading the pressurized hydraulic fluid from the engine oil supply to the rotating, hydraulically actuated camshaft phasing assembly.
In order to address the shortcomings of such pressurized hydraulic fluid actuated camshaft phasing assemblies, it has also been proposed to use a closed hydraulic circuit in an engine camshaft phaser in which the hydraulic circuit employs a magneto-rheological fluid as a hydraulic pressure medium. This is disclosed in PCT/GB02/05464. In this disclosure, a camshaft phaser is provided having a rotor attached to the camshaft and a stator connected to the crankshaft via a timing chain and gear. Hydraulic working chambers are located on opposing sides of two vanes which extend from the rotor into working chambers defined in the stator. The magneto-rheological fluid flows freely between the chambers via passages which connect opposing chambers. The flow of the magneto-rheological fluid through the passage is controlled by the selective application of a magnetic field.
Magneto-rheological fluids are materials that respond to an applied magnetic field with a change in their properties. In this case, the application of a magnetic field causes the magneto-rheological fluid to increase greatly in viscosity to a virtual solid state. Magneto-rheological fluids have a very fast response time and their flow properties change within milliseconds of application or removal of the magnetic field. One example is MRF-122EG from Lord Corp.
In the case of a camshaft phaser, as the rotor, which is typically connected with the camshaft, fluctuates in position relative to the stator within a defined rotational angle based on the travel available to the vanes in the working chambers, when a desired position is achieved, the electromagnet is actuated and the position of the rotor is hydraulically locked relative to the stator due to the passage between the chambers being blocked by the change of state of the magneto-rheological fluid in response to the magnetic force being applied.
DE 102 33 044 A1 also discloses a magneto-rheological fluid actuated camshaft adjuster in which passages between the working chambers are provided via openings defined through the vanes of the rotor. However, in these known prior art devices, it is necessary to provide current to the electromagnet as it rotates with the camshaft adjuster. Further, these prior art solutions change from a low viscosity fluid state when no electromagnetic force is applied to a nearly solid state when the electromagnetic force is applied, making controlled shifting of the phase position of the camshaft relative to the crankshaft more complicated.